This invention generally relates to improving electrical connections to materials with high-dielectric-constants, such as in the construction of capacitors.
Without limiting the scope of the invention, its background is described in connection with current methods of forming electrical connections to high-dielectric-constant materials, as an example.
The increasing density of integrated circuits (e.g. DRAMs) is increasing the need for materials with high-dielectric-constants to be used in electrical devices such as capacitors. The current method generally utilized to achieve higher capacitance per unit area is to increase the surface area/unit area by increasing the topography, such as in trench and stack capacitors using SiO2 or SiO2/Si3N4 as the dielectric. This approach becomes very difficult in terms of manufacturability for devices such as the 256 Mbit and 1 Gbit DRAMs.
An alternative approach is to use a high permittivity dielectric material. Many perovskite, ferroelectric, or high-dielectric-constant (hereafter abbreviated HDC) materials such as (Ba,Sr)TiO3 (BST) usually have much larger capacitance densities than standard SiO2xe2x80x94Si3N4xe2x80x94SiO2, capacitors. Various metals and metallic compounds, and typically noble metals such as Pt and conductive oxides such as RuO2, have been proposed as the electrodes for these HDC materials. To be useful in electronic devices, however, reliable electrical connections should generally be constructed which do not diminish the beneficial properties of these high-dielectric-constant materials.
As used herein the term high-dielectric-constant means a dielectric constant greater than about 150. The deposition of an HDC material usually occurs at high temperature (generally greater than about 500xc2x0 C.) in an oxygen containing atmosphere. The lower electrode structure should be stable during this deposition, and both the lower and upper electrode structures should be stable after this deposition. There are several problems with the materials thus far chosen for the lower electrode in thin-film (generally less than 5 um) applications; many of these problems are related to semiconductor process integration. For example, Ru is generally not a standard integrated circuit manufacturing material, and it is also relatively toxic. Pt has several problems as a lower electrode which hinder it being used alone. Pt generally allows oxygen to diffuse through it and hence typically allows neighboring materials to oxidize. Pt also does not normally stick very well to traditional dielectrics such as SiO2 or Si3N4, and Pt can rapidly form a silicide at low temperatures. A Ta layer has been used as a sticking or buffer layer under the Pt electrode, however during BST deposition, oxygen can diffuse through the Pt and oxidize the Ta and make the Ta less conductive. This may possibly be acceptable for structures in which contact is made directly to the Pt layer instead of to the Ta layer, but there are other associated problems as described hereinbelow.
Other structures which have been proposed include alloys of Pt, Pd, Rh as the electrode and oxides made of Re, Os, Rh and Ir as the sticking layer on single crystal Si or poly-Si. A problem with these electrodes is that these oxides are generally not stable next to Si and that these metals typically rapidly form silicides at low temperatures (generally less than about 450xc2x0 C.).
One difficulty with the previous solutions is that they generally utilize materials (e.g. Ru) which are unusual in a semiconductor fab. Another difficulty is that a relatively good dry etch for Pt or RuO2 does not yet exist. As another example, there currently does not exist a commercial chemical vapor deposition process for Pt or Ru. In addition, Pt is normally a fast diffuser in Si and therefore can cause other problems. Also, most of the proposed electrode structures require several additional process steps which can be uneconomical. For example, there currently does not exist a commercial chemical vapor deposition process for Pt or Ru, nor a commercial dry etch for RuO2.
Generally, the instant invention uses a lightly donor doped perovskite as the electrode in a thin-film microelectronic structure. An electrode buffer layer may also be used as a sticking layer and/or diffusion barrier and/or electrical connection, if needed. A lightly donor doped perovskite generally has much less resistance than undoped, acceptor doped, or heavily donor doped HDC materials. The bulk resistivity of a typical lightly doped perovskite such as BaTiO3 is generally between about 10 to 100 ohm-cm. Also, this resistivity may be further decreased if the perovskite is exposed to reducing conditions. Conversely, the perovskite can achieve a high resistivity (about 1010-1014 ohm-cm) for large donor concentrations. The amount of donor doping to make the material conductive (or resistive) is normally dependent on the process conditions (e.g. temperature, atmosphere, grain size, film thickness and composition). There exists a large number of perovskite, perovskite-like, ferroelectric or HDC oxides that can become conductive with light donor doping.
The deposition of the lightly donor doped perovskite lower electrode may be performed in a slightly reducing atmosphere in order to minimize the oxidation of the layer(s) underneath it. The subsequent deposition of the HDC dielectric material can require very oxidizing conditions, and the lightly donor doped perovskite lower electrode slows the oxidation rate of the layer(s) underneath it, thus inhibiting the formation of a substantially oxidized continuous resistive contact layer. Another benefit of this electrode system is that the lightly donor doped perovskite lower electrode does little or no reduction of the HDC dielectric material.
The disclosed structures generally provide electrical connection to HDC materials while eliminating many of the disadvantages of the current structures. One embodiment of this invention comprises a conductive lightly donor doped perovskite layer, and a high-dielectric-constant material layer overlaying the conductive lightly donor doped perovskite layer. The conductive lightly donor doped perovskite layer provides a substantially chemically and structurally stable electrical connection to the high-dielectric-constant material layer. A method of forming an embodiment of this invention comprises the steps of forming a conductive lightly donor doped perovskite layer, and forming a high-dielectric-constant material layer on the conductive lightly donor doped perovskite layer.
These are apparently the first thin-film structures wherein an electrical connection to high-dielectric-constant materials comprises a conductive lightly donor doped perovskite. Lightly donor doped perovskite can generally be deposited and etched by effectively the same techniques that are developed for the dielectric. The same equipment may be used to deposit and etch both the perovskite electrode and the dielectric. These structures may also be used for multilayer capacitors and other thin-film ferroelectric devices such as pyroelectric materials, non-volatile memories, thin-film piezoelectric and thin-film electro-optic oxides.